A known technique for fabricating composite material parts, in particular parts made of thermostructural composite materials such as carbon-carbon composites or ceramic matrix composites, consists in making preforms of the parts and in densifying the preforms in a chemical vapor deposition or infiltration oven. Preforms for parts are generally constituted by fibrous textures or fabrics. The preforms, optionally held in shape by tooling, are inserted into an oven into which there is admitted a gas whose composition is selected as a function of the nature of the matrix of the composite material. Under predetermined conditions of temperature and pressure, the gas diffuses throughout the pores in the preforms and, by decomposition of one of its components or by reaction between a plurality of its components, it leaves behind a solid deposit that progressively densifies the preforms by filling up their pores and thus forming the matrix.
Since chemical vapor deposition or infiltration takes place under low pressure, it is necessary to provide pumping means connected to the oven. An industrial installation for chemical vapor infiltration and operating with a water ring pump is described in document WO 87/04733.
The gaseous reaction products and/or the residues of the gas as extracted from the oven by pumping may give rise to difficulties because they are toxic or corrosive, or because they tend to form unwanted deposits.
This applies in particular when fabricating parts made of composite materials in which the matrices are at least mainly constituted by a ceramic such as a silicon carbide (e.g. SiC), or, more particularly, when the matrix includes at least one ceramic such as boron carbide (B.sub.4 C) or a silicon-boron-carbon (Si--B--C) ternary system as described in document EP-A-0 483 009.
The precursors used in the gas then generally include gaseous halides, in particular gaseous chlorides, which, in addition to being corrosive and toxic, can also form considerable solid deposits on leaving the oven. This applies in particular to boron trichloride BCl.sub.3, a precursor for the element boron.
In the above-mentioned installation that uses a water ring pump, it has thus been observed by the Applicant that the presence of moisture at the inlet to the pump gives rise to solid deposits because of reaction with the gaseous chlorides, thereby progressively and inescapably clogging the pumping pipework. It is then necessary to perform frequent maintenance, or indeed replacement, of the components in the pumping device, which operations require great precautions to be taken because of the unwanted solid deposits are frequently of an unstable nature.